Hot dip galvannealed steel ("HDGA") is extensively used in the automotive industry because it has superior weldability, paintability and cosmetic corrosion resistance after painting, as compared to, for example, pure zinc coated steel. Galvannealing process conditions must be controlled tightly in order to produce high quality HDGA products. One very important process parameter which affects HDGA product quality greatly is the galvannealing temperature.
Reliable temperature measurements enable more complete characterization of the galvannealing process, so that a dynamic model can be developed to control the process. Process variations that affect surface temperatures of galvanneal during processing directly impact the quality of the end-product steel. Several parameters, such as sheet thickness, sheet speed, and power to the induction furnaces, each of which affects the temperature, will vary during production. Hence, if they can be controlled in real time, it will be possible to improve the quality of the end product. Better technology for temperature monitoring can lead to enhanced product quality and consistency, and reduce process spoilage.
It is therefore desired to develop a system for the in-process measurement of galvanneal steel strip temperatures. The subject invention was developed specifically for galvanneal strip temperature measurement, but is equally applicable to other environments.
Infrared pyrometry is sometimes used as a temperature indicator for galvanneal processing. However, the accuracy of pyrometry depends directly on a surface's emissivity. Because the emissivity of the molten layer of zinc on the sheet varies rapidly during the process, pyrometry can introduce considerable uncertainty.
We are aware of no accurate in-process means of measuring galvanneal steel surface temperature which is independent of emissivity. In many steel monitoring applications, high temperatures, unknown emissivities, moving and molten surfaces, hazy viewpaths, and environments inimical to most types of instrumentation combine to undermine accurate thermometry. The subject invention addresses these problems by implementation of a unique system based on thermally sensitive luminescence.
Thermographic phosphors are relatively inert, doped ceramics that emit light with a distinctive spectral distribution when suitably excited by an energy source such as an electron beam, x-ray emission, or ultraviolet light. If the source is transient or pulsed, then the luminescence will have a characteristic decay rate or decay lifetime, i.e., persist for a characteristic duration after cessation of the excitation radiation. These luminescence properties change with temperature. These changes are independent of the emissivity of the surface to which the luminescing material is attached. Other related luminescence properties, such as, for example, emission intensity or phase shift, may be exploited to ascertain temperature. However, the present invention is based on decay rate measurements that do not require intensity calibration, because they therefore tend to retain calibration better than other measurements, such as those required for other approaches.
During the decay period, the brightness of the luminescence decreases exponentially. The time it takes the brightness to fall by a factor of 1/e (wherein e is the base of the natural logarithm) is termed the "decay time." FIG. 1 illustrates this phenomenon for a representative phosphor. Note that the rate of decrease is more rapid at the higher temperatures. Provided the decay is described by a single exponential and there are no chemical reactions altering the phosphor, the decay time is a single-valued function of temperature over a wide and useful range of temperatures.
It is well known to measure temperature using luminescence decay rates. For many years, phosphors have been applied to various substrates or in various environments in order to determine temperatures that were otherwise impractical to measure. However, we are aware of no system which addresses the problems encountered in environments such as a galvannealing process line. In galvannealing, it is not practical to apply a phosphor to the material at the beginning of the process and later test its luminescence, because the steel surface must be clean as it enters the zinc bath. Thus, the phosphor cannot be applied until after the steel exits the bath. This point in the process, after the steel has exited the zinc bath, is where it is particularly important to obtain a temperature reading. Therefore, it would be helpful if the phosphor could be deposited onto the galvanneal sheet after it has exited the bath.
Adding to the difficulty of depositing the phosphor to the steel at this stage of the process, is that it is difficult to predict exactly how the sheet will be moving. Not only do speeds vary, but the sheet may not be uniformly thick or flat at this juncture, and will probably be wobbling as it moves.
Thus, there is a need in the art for a phosphor-based thermometry system in which the phosphor is deposited on a moving substrate to be monitored.
There is a further need for such a thermometry system which can accommodate a substrate that is moving unpredictably.
There is also a need for such a thermometry system which can deposit the phosphor in a harsh environment to a hot substrate.